Hard code freeze break approval plea for Evolution

[Milan Crha <mcrha redhat com>]

        Hello,
after a lengthy investigation around changes in WebKitGTK+ 2.25.x [1],
there was found a way to workaround the problem [2] for now, which
gives time to look for better solution in the future.

The change on the Evolution side is as simple as setting an environment
variable in its main(), to let WebKitGTK+ know the old behavior is
required, something like:

   g_setenv ("WEBKIT_USE_SINGLE_WEB_PROCESS", "1", FALSE);

This won't break older WebKitGTK+, because it doesn't know about that
new environment variable, and it'll help with the new WebKitGTK+. The
variable addition will be part of 2.25.92 release of WebKitGTK+.
        Thanks and bye,
        Milan

[1] https://bugs.webkit.org/show_bug.cgi?id=201033
[2] https://gitlab.gnome.org/GNOME/evolution/issues/587